For example, Patent Document 1 shows an engine, in which a combustion chamber is segmented into a central combustion chamber and a main combustion chamber by a cavity recessed at the lower surface of each cylinder head and a protrusion raised at the top surface of a piston to increase the theoretical thermal efficiency of a spark ignition gasoline engine. The combustion chamber has a high compression ratio of about 16 as a whole. The central combustion chamber contains a relatively rich air-fuel mixture and the main combustion chamber contains a relatively lean air-fuel mixture so that the combustion chamber contains a lean air-fuel mixture as a whole.
For example, Patent Document 2 teaches that a surface segmenting a combustion chamber of an engine is made of a cellular heat insulating material in view of improving the thermal efficiency by reducing cooling loss. In Patent Document 2, the engine has a compression ratio of 16.